Sponge glass seal for wave guides



E. G. UNDER 2,670,462

SPONGE GLASS SEAL FOR WAVE GUIDES Feb. 23, 1954 Filed May 28, 1949 '111111111111111'll'lrqillll'l'arlp a Patented Feb. 23, 1954 SPONGEGLASS SEAL FOR WAVE GUIDES iErnest LG. Linden iPrinceton, 5J., assigner to .:Radio-,6orporation of America, a corporation @of Delaware A4A.pplication'May'28, 1949,'*Serial No. 96,091

.'132Claixns. il

*This invention relatesvv to improvements inI elec- "trmagnetic K' permeable structures in transmis- *sion--systems such as conductor-spacing bushings and lgas-tight #bushings for coaxial lines and R. F.-windows' for wave-guide systems.

-More particularlyit relates to-improvements inssuch bushings and Windows `whereby the impedance-discontinuities Whichthey produce are substantiallyl negligible.

Itrisk Well known f'thatithere-is wide usage in -theelectrical arts; and inparticular inl those in- 'volv'ing'the useof cOaXiallineS, wave guides,'and "ultra lhigh frequency transmission systems oper- --ating in Avacuo or under'pressure, to `employ 'vbushings'and lwindows-made of `good quality insulating `material such as glass which Will not cause Ik@excessive losses and-which, -whennecessary; will be capable of" being bonded to` the inner and outer Aconductors 'of a coaxial f line or' to the walls of -a' waveguide or Acavity resonator to'form a gaseti'ght'seal.

Sometimes where a gas-tight sealf-isnot Vrequired, a center conductor YofA a coaxial 'system visv supported on a yquarter-wave#length stub which uat Aan yexact frequency will act Aas yan '-'excellent insulator -and which, -due toits -being made of metal,v is `vstructurally very strong. Of'course this will not be of any use where the supportimust Yalso 'act as va fgas'seal. Moreover this vtype of `center conductor supportlis not suitable for a system operating overa wide bandv -of-"frequencies *Hencebusliings of non-conductive materials are muchmore commonly used. However, Yeven* this v-typecf Acenter vconductor "support (and/ or intercon'ductor seal) is not satisfactory Awhen the transmission system fis Vto be utilized for a wide A'bandfof Vfrequencies This is because-of the'ffact that inthe prior'art-agoodinsulator of atype Ksuitable for forming a vseal usually has Vhad a high dielectric constant and'therefore "has prozduced a *seriousldiscontinuity--ini the transmission system. Though theeffects of thediscontinuity rcou'lclbe minimizedby'loeatingthe bushing` at a ipos'ition'where'fa; voltage null' 'will `occur for apar- "ticular i'frequency;iitf'couldrnot bei soi located if the 'i system were required to 4operateover )a wideband. -fObv-iously this .is vequally =true of `'a ldielectric window.

'Thusjit hasfbeen lquite usual forvitreousv bush- `vl`ingeseals and windows -to cause lreflections 'in ltransmission"systemsusing them. In the case `of bushings .various expedients have -beentried for tooth offthe :conductors-.pf 'ja coaxial fixture the electric field is radial.

*having different kdielectric constants. isthiswasteful of Vmaterial but usually it4 results f2 a Aglass bushing was'to be locatedxso thattthe characteristic impedance of 'that :section .would be equal to that of other sections :iin fwhichthe only dielectric Was air'with its Inuchflower-wdielectric'constant.

In practice the use'of under-.cutting hasaproven Very unsatisfactory. The required arnount :of

under-cutting has usually been determined -by "the use of 'formulae `which `actually aretcorrect l0l lend-eifects"have been `disregarded fand :thecasonly-for long lines. In deriving these'formulae,

lsuinption made that over `most offa coaxialline For f a. long #line this is justified because of theffact'that 4the fringing fields which exist at its endsw`i1lfonly aifectthe `radial field configuration? for-'relatively .small dis- "tances back from the ends. "Howeverfasection which is under-cut to rreceive a bead'is soshort thatend effectsfare felt over most: of its length.

Thus, calculations made on the basis-of available formulae yield -inaccurate results. -Moreovenirit hasA been found that where the amountoffundercutting is 'determined empirically, inorder .tocor- Arect for f the f inaccuracies of 1 these f formulae, the results lobtainedware correct Yonly-"for a'narrow "frequency band, ige., the'v beadI causes'irnpedance VIdiscontinuities if "the vsystem is made .to `operate on either side of that band. VAn `added*disadvantage is lthatthis typeof constructionris-'ex- 'pensiveas it'requires special machinefoperations lin forming Vone or both of the conductors aswell V'as the-use of -heavy materials torperniit.t-hese machine operations. 'While thisexpense may'` be 'l tolerated if it is incurred'only`fortheffxtures=and connectors of a system, it :assumes serious proportions Where this. construction is usedforlong v-vtransinission line runs also required ini'the :sys- "tem. Moreoven this type lof construction vdoes not lend itself to use in long lines.

Sometimes the 4expedient is practicedl of using tem to maintain the spacing between the? inner and 4outer conductors'while -at the Sametime avoiding repeated transitions between insulators 'Not only inetron) and'to'use very thin windows. Theirst of these" has the V,disadvantage offoperatin'gsatls- 4ffa'ctori'lyoverr only `a 'limited ban'dand ltl'iesecond khas *the vdisadvantage of poor 'mechanical strength. In some broad band magnetrons it has been the practice to make the windows of mica since this material is elastic and therefore, unlike thin glass, it can yield to atmospheric pressure without fracturing.

It is an object of the present invention to devise an electro-magnetic permeable structure for use in a high frequency transmission system which will have a dielectric constant of substantially l so that, Without the use of expedients such as the under-cutting of one or more of the conductors of a coaxial line where it is a coaxial transmission system, the system Will have substantially the same characteristic impedance at points where it is insulated with gas, such as air, as at the point where said structure, such as a bushing, a seal, or a window, is employed.

It is a further object of this invention to provide an electro-magnetic permeable structure of the kind set forth above which is in the form of a bushing for a coaxial transmission system and can be bonded tor the metallic conductors thereof to produce a gas-tight seal.

It is a further object of the present invention to devise a transmission system which is prmarily insulated by gas, such as air, but also utilizes bushing seals, of the kind to be described hereinafter, in an arrangement whereby the system may be pressurized sufliciently to raise the dielectric constant of the gas to the value, only slightly higher than l, of said bushings, whereby the system will be substantially free of any reflective discontinuities.

It is a further object of the present invention to devise a coaxial line connector including a bushing-seal which is an electro-magnetic permeable device of the kind set forth above and which is embodied in an arrangement for protecting the seal from damage during coupling and uncoupling of the connector.

It is a further object of this invention to devise a dielectric window having good dielectric and structural strength, and a dielectric constant close to unity whereby it will not produce any appreciable impedance discontinuity in a gas insulated wave guide system over a wide band of operating frequencies.

Other objects, features and advantages of this invention will be apparent to those skilled in the art from the following description of several illustrative embodiments of the invention and from the drawing in which:

Fig. 1 represents the evacuated envelope of a known electron discharge device ofthe velocitymodulatd beam type to show the use of a gastight insulating bushing-seal according to the present invention for the output connector thereof Fig. 2 shows o'ne type of prior art for the output connector of a device such as that represented in Fig. 1;

Figs. 3, 4 and 5 illustrate further `embodiments of the present invention; and

Fig. 6 illustrates an embodiment of the present invention in which the electro-magnetic permeable structure is an R. F. window for a magnetron.

Fig. 1 shows a velocity modulated inductive output discharge device I of a kind now well known in the art. It comprises an input or buncher cavity 3 and an output cavity 5. These cavities are mechanically connected together in a known manner tov enclose between them a driftspace I through which a beam of electrons passes from the input cavity 3 to the output cavity 5.

To complete the vacuum enclosure for the discharge device I there are two dome-like closures 9, II which are respectively joined to the cavities 3 and 5 so that together these four elements 3, 5, 9, II enclose an evacuated space. Supported within closure 9 is a conventional gun structure I3 for producing a beam of electrons and directing it into the drift-space 'I through a pair of parallel grids I5, |51 of the input cavity 3. A collector electrode I1 is positioned within the closure II to receive the beam after it has passed out of the drift space I through a pair of grids I9, |91 of the output cavity 5. All of the structure thus far described is entirely in accordance with usual and known practice and for this reason the device I will not be described herein in any further detail.

An output coupling 2l includes an inductive loop 22 which is positioned within the output cavity 5 in a region of large magnitude radio frequency magnetic eld. A portion of the coupling 2 I, which is located outside of the discharge device I, constitutes a coaxial line connector including a cylindrical outer conductor 23, an

Vinner conductor 25 and a bushing-seal 21. The

connector shown in Fig. l illustrates the fact that neither conductor of a bushed element of a coaxial transmission system according to the present invention needs to beunder-cut in order to attain for the element the same characteristic impedance as that of air-insulated portions of the system. This is made possible by making the bushing-seal 2'! of sponge-glass formed with a small enough ratio of glass to air, not less than .01 and not greater than .1, to cause its dielectric constant to be close to unity. I have ascertained that despite the cellular structure of this material, bushings which are made of it in lengths not much greater than those of ordinary glass beads may be bonded to the conductors to form a seal capable of preserving a hard vacuum or holding considerable gas pressure.

Fig. 2 illustrates a conventional type of construction for the output connector of a device as in Fig. 1 in which a solid glass bead is employed in combination with under-cutting of the conductors.

It is possible to 'bond the sponge-glass bushing 2l to each of the conductors 23 and 25 by heating the conductor until the part of the bushing adjacent to it flows onto it and to do this Without causing the entire bushing to collapse into a small solid mass of glass. This is possible, despite the very small volume of actual glass in the bushing, because of the fact that both the glass and the air spaces formed within its sponge-like structure have very low thermal conductivity.

However, there is described below a method of reducing any possibility of melting the entire bushing in bonding it to the conductors. This method can be best understood by reference to Fig. 3.` As shown therein an outer conductor 3I of a coaxial line has a thin lm 33 of low melting point glass fused to its inner surface in the region thereof where a sponge glass bushing-seal 39 is to be located. It will be understood that the term low melting point glass means a glass having a melting point lower than the melting point of the sponge glass bushing, whereby the lm 33 can be fused without melting the sponge glass bush- .ing-

together the bushing and the two conductors 3l, 35, the bushing is placed over the nlm 31 on the inner conductorf35 and radio frequency heating is induced in the inner conductor. This will not cause heat to be directly generated in either the bushing 39 or the nlm 31. However, heat induced in the inner conductor 35 will be thermally conducted to the nlm 31 and to adjacent portions of the glass of the bushing 39 and after a predeterminable length of time will fuse them together and to the conductor. Thereafter the inner conductor 35 and the bushing 39 are inserted into the outer conductor 3| until the bushing is surrounded by the nlm 33. The outer conductor 3| may be heated directly, as by a gas flame, or by induction until the iilm 33 flows sufliciently to bond the bushing to the outer conductor.

As is known, the amount of heat which Will be absorbed by the bushing 3S during the heating of one of the conductors, 3i or 35, will depend not only upon the conductivity of the material oi which the bushing is formed and of the air in its interstices but also upon the period of tirne over which heat is applied to, or generated in, the conductor. Accordingly, there is an added advantage to be gained in using radio frequency heating, since, as is Well known, it is possible by so doing to generate intense heat in an extremely short and predeterminable time. 1f the heating time is short enough, considerable success may be attained by following the process described above even if the glass iilms 33 and 31 are formed of the same glass as the bushing rather than of low-melting-point glass. This is partly due to the fact that in practice the nlm will melt before the bushing is dangerously heated since it is in such intimate contact with the metal in which the heat is generated.

If it `is desired to mechanically strengthen the bushing 39, for example, to strengthen one of its ends to protect it against damage from lateral thrusts which may be brought to bear against the end of the inner conductor 35 which extends out of the bushing into free space, a solid glass disc 4l may be dropped into place as shown in Fig. 3 and fused to the bushing 39 and the two conductors by the direct application of heat. Preferably disc 4I should be a loW-melting-point glass. Even if the disc 4| is made thin enough so that it will not produce any noticeable discontinuity v in the line it will have considerable structural strength due to its being backed up by the bushing 39 in the manner shown herein.

Fig. 4 shows .an alternate construction for mechanically strengthening the termination of a coaxial line employing a bushing according to the present invention. In Fig. 4 the sponge-glass bushing 39 is positioned a little distance back from the end 43 of the coaxial line. For this short distance another bushing 5 is used which, while it also has a sponge-like structure (to cause it to have a low-dielectric constant), is different from bushing 39 in that it is made of a more resilient material. Such a material is sponge polystyrene or "fstyrol" If the bushing 45 is suiiiciently long it will ordinarily absorb enough of the energy from any accidental lateral thrusts on the terminal end of the inner conductor 35 to prevent damage to any of the air cells located near the center of the sponge glass bushing 39. However, if it is desired to provide further protection for the bushing 39 va short section 41 of exible inner conductor may be v.inserted between -the portion of innerconductor 35 which is sealed within the bushing 39 and the portion thereof 6 which fis carried within the bushing 45 and extends from it at the end 43. This section 41 may be composed of a bundle of thin Wires enclosed in a braided sheath such as the type of sheath used for shielding. The sheath should have the same external diameter as the inner conductor 35.

Fig. 5 illustrates a section of a coaxial transmission line system employing sponge glass gastight bushing-seals according to the present invention and a means such as a pump 5I for pressurizing the gas, such as air, which insulates the remainder of the system. It is known that it is advantageous to pressurize a transmission system to increase the dielectric strength of the gas rinsulation thereof and that sometimes an incidental advantage arises in that the air is dried as it is pressurized thus avoiding greatly lowered dielectric strength in damp Weather. In addition to all of these advantages there is another which .is oiered by the embodiment shown in Fig. 5 `as a result of the fact that the dielectric constant of the bushing-seals 39 is so close to unity. This is that it is possible to obtain the same characteristic impedance at all points and for all elements of the transmission system since the pressurizing will be adequate to raise the dielectric constant of the gas insulation by the small amount by which it is exceeded at normal pressure by that of the sponge dielectric. By proper selection of the insulating gas it is possible either to control What variations in dielectric constant are brought about by variations in pressure or to eliminate the need for pressurizing. For example, as is known, ammonia gas has a higher dielectric constant than air and the increases in its dielectric constant which occur with increases in pressure are greater than those which occur for corresponding increases in the pressure of air.

Fig. 6 illustrates an embodiment of the present invention as an R. F. window for the output of a plural cavity magnetron. In order to simplify the drawing andto conserve space only that portion of the magnetron is shown which is necessary to indicate the manner in which the window is installed. The cathode for the magnetron is represented at 6l. As is known, the cathode of such a magnetron is surrounded by a plurality of radial resonant spaces or cavities. In 6 only one of these is shown. It is the output cavity for the magnetron and is shown at 63. Cavity 63 is formed within a portion of the curved outside wall of the magnetron, the two varies 61 and 69 and the circular end closures of the magnetron envelope (not shown). Cut through the curved outside wall of the output cavity 63 there is a narrow slot 1l for coupling energy from the magnetron to a wave guide transmission system. The slot feeds a transitional section of wave guide 13 which includes a tapered portion 15 having its small end connected to the curved wall 65 over the slot 1l and its large end connected to a rectangular portion 11u/nich over all of its length isof the same cross-sectional 1` dimension asa wave guide systemto 'oe fed from in adielectric@angemeldete unity.. .For this reason no 'appreciable' discontinuity will be o ffered by the window 8l even though' it is very thick. In fact, this will be true even if most of the transitional section is solidly filled with this material. If desired a thin surface film 83 of solid glass may be employed to protect the window and to improve the seal. The inside walls of the transitional section 13 carry thin films 85 of solid low-melting-point glass -for bonding the window 8l to the section. The reasons for the use of this lm are the same as those mentioned above in describing other embodiments of this invention.

While I have indicated the preferredembodi ments of my invention of which I am now aware and have also indicated only one specic application for which my inventionv` may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for whichlit is employed without departing from the scopel of my invention as set forth in the appended claims.

What I claim as new is:

1. A high frequency transmission system including a conductive metallic structure a portion of which defines a passage positioned in the system to permit the transfer of electromagnetic energy from one part of the system to another through the structure, andA an electromagnetic energy permeable sponge glass insulator located in said passage and bonded to said portion in gas-tight relation to seal said passage without preventing said transfer of energy, said insulator being formed to have 'a sponge-like structure in which the ratio of the ,volume 'of solid material to that of interstitial space has a value between .O1 and .1 for which the insulator has a dielectric constant close to unity.

2. A high frequency evacuated conductive structure having an aperture therein and glass means permeable to electromagnetic .energy sealed across said aperture in gas-tight relation, said means comprising a sponge-glass insulator in which the ratio of the volume of solid glass to that of the interstitial space has a value between .01 and .1 for which theinsulator has a dielectric constant close to unity. d

3. A section of coaxial transmission line comprising an outer cylindrical conductor, an inner cylindrical conductor, and an insulating bushing for supporting theinner conductor in a coaxial relationship with said outer conductor, the bushing being made of sponge glass in which the ratio of the volume of solid glass to lthat of interstitial space has a value between .01 and ..1 for which the bushing has a dielectric constant close to unity, said sponge glass bushing being bonded to said conductors in gas-tight relation.

4. A section of coaxial transmission line comprising an outer hollow conductor, an inner conductor, an insulating bushing for supporting the innerY conductor within said outer conductor, lthe bushing being made of glass formed tojhavej a sponge-like structure in which the ratio of solid glass to that of interstitial space has a value between .01 and `1 for which the structure has a dielectric constant close to unity, a thin film of solid glass fused to one of thesurfaces of the bushing and to a portion of the surface of one of the conductors to bond thernpltogether. u

. -5. A coaxial transmissionjsystem, comprising an outer hollow conducthrfahflnner conductor, an insulating bushingfor 'supportinga portion of the inner conductor within "said outer conductor near an end thereof, the bushing being made of sponge glass having a dielectric constant close to unity, a second bushing adjacent to said first-mentioned bushing and between it and said end of the outer conductor, the second bushing being made of an insulating material which has, as a solid, a dielectric constant substantially greater than one but is formed into a sponge-like structure suiciently diliused to have a dielectric constant close to unity, the second bushing being long enough and said insulating material of which it is made being re-v silient enough to absorb energy from lateral thrusts exerted upon the inner conductor so as to protect the adjacent sponge-glass bushing :from damage thereby.

6. A coaxial transmission system comprising an outer hollow conductor, an inner conductor, an insulating bushing for supporting the inner conductor within said outer conductor, the bushing being made of sponge glass having a dielectric constant close to unity, a second bushing adjacent to said first-mentioned bushing, the second bushing being made of an insulating material which has, as a solid, a dielectric constant sub-g stantially greater than one but is formed into a sponge-like structure suiiiciently diffused to have a dielectric constant close to unity, the second bushing being long enough and said insulating material of which it is made being resilient enough to absorb energy from lateral thrusts exerted upon the inner conductor so as to protect the adjacent sponge-glass bushingfrom damage thereby, and said inner conductor including a flexible section which is carried within the second bushing as a portion of the inner conductor surrounded thereby and extends between the portion of the inner conductor which is surrounded by the first-mentioned bushing and the remaining portion of the inner conductor which is surrounded by the second bushing.

7. A coaxial transmission system including an outer cylindrical conductor and an inner cylindrical conductor, a first bushing for supporting the inner conductor in a coaxial relationship with the outer conductor, said bushing being composed of sponge glass in which the ratio of the volume of solid glass to that of interstitial space has a value between .01 and .1 for which the bushing has a dielectric constant close to unity and being bounded to the conductors to provide a gas-tight seal at one point in the system, a second bushing similar to the rst bushing and bonded between said conductors to form a gas-tight seal at another point in the system and a non-conductive gas occupying the space between said conductors and said bushings at a predetermined pressure at which the gas has a dielectric constant substantially equal to that of said bushings.

8. A coaxial transmission system comprising an outer cylindrical conductor, an inner cylindrical conductor, and an insulating bushing ufor supporting the inner conductor in a coaxial relationship with said outer conductor, the bushing being `made of sponge glass in which the ratio of the outer'diameter to snugly within theinner ccnductor, said disc having one Vofits surfaces fused to an end of said bushing and its outer and inner edges fused respectively to said outer and inner conductors.

9. A high frequency system comprising an evacuated conductive closure which includes a window portion providing an electrical opening to permit the transfer oi electro-magnetic energy between the inside and the outside of the closure, the window portion being made of glass formed to have a sponge-like structure in which the ratio of the volume of said material to that of interstitial space has a value between .01 and .1 for which the insulator has a dielectric constant close to unity, the Window being bonded around its periphery to the closure in gas-tight relation.

l0. A high frequency transmission system including a hollow conductor and glass means permeable to electromagnetic energy sealing said holloW conductor in gas-tight relation, said means comprising a sponge glass insulator in which the ratio of the volume of solid glass to that of the interstitial space has a value between .01 and .1 for which the insulator has a dielectric constant close to unity.

11. A high frequency transmission system according to claim 10, wherein said sealing means includes a thin nlm of solid glass interposed between and fus-ed to said hollow conductor and said insulator.

12. A section of coaxial transmission line according to claim 3, wherein said sponge glass 10 bushing is bonded directly to said outer and inner conductors.

13. A high frequency transmission system aeccrding to claim 19, wherein said sponge glass insulator is bonded directly to said hollow conductor.

ERNEST G. LINDER.

References Cited in the file 0f this patent UNITED STATES PATENTS Number Name Date 1,859,399 Green May 24, 1932 2,929,421 Green et al Feb. 4, 1936 2,216,016 Hobart Sept. 24, 1940 2,456,653 Snow et al Dec. 21, 1948 2,478,573 Dorgeio Aug. 9, 1949 2,490,523 Lemmens Dec. 6, 1949 2,499,645 Sorg Dec. 13, 1949 2,533,771 Feenberg Jan. 23, 1951 2,579,324 Koek Dec. 18, 1951 FOREGN PATENTS Number Country Date 613,463. Great Britain Nov. 29, 1948 OTHER REFERENCES Publication Sintered Glass by E. G. Dorgelo, in The Glass Industry, pub. by Ogden Publ. Co., of N. Y.; vol. 27, No. 7, July 1946, pp. 347, 348, 364 and 363-370. 

